ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-8411-2017Understanding the primary emissions and secondary formation of gaseous
organic acids in the oil sands region of Alberta, CanadaLiggioJohnjohn.liggio@canada.caMoussaSamar G.WentzellJeremyDarlingtonAndreaLiuPeterLeitheadAmyHaydenKatherineO'BrienJasonMittermeierRichard L.StaeblerRalfWoldeMengistuLiShao-Meng1Air Quality Research Division, Environment and Climate Change Canada,
Toronto, Ontario, M3H 5T4, Canada2National Research Council Canada, Flight Research Laboratory, Ottawa, K1A
0R6, CanadaJohn Liggio (john.liggio@canada.ca)11July20171713841184279March201713March201719May20175June2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-chem-phys.net/17/8411/2017/acp-17-8411-2017.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/17/8411/2017/acp-17-8411-2017.pdf

Organic acids are known to
be emitted from combustion processes and are key photochemical products of
biogenic and anthropogenic precursors. Despite their multiple environmental
impacts, such as on acid deposition and human–ecosystem health, little is
known regarding their emission magnitudes or detailed chemical formation
mechanisms. In the current work, airborne measurements of 18 gas-phase
low-molecular-weight organic acids were made in the summer of 2013 over the
oil sands region of Alberta, Canada, an area of intense unconventional oil
extraction. The data from these measurements were used in conjunction with
emission retrieval algorithms to derive the total and speciated primary
organic acid emission rates, as well as secondary formation rates downwind of
oil sands operations. The results of the analysis indicate that approximately
12 t day-1 of low-molecular-weight organic acids, dominated by
C1–C5 acids, were emitted directly from off-road diesel vehicles
within open pit mines. Although there are no specific reporting requirements
for primary organic acids, the measured emissions were similar in magnitude
to primary oxygenated hydrocarbon emissions, for which there are reporting
thresholds, measured previously (≈ 20 t day-1). Conversely,
photochemical production of gaseous organic acids significantly exceeded the
primary sources, with formation rates of up to ≈ 184 t day-1
downwind of the oil sands facilities. The formation and evolution of organic
acids from a Lagrangian flight were modelled with a box model, incorporating
a detailed hydrocarbon reaction mechanism extracted from the Master Chemical
Mechanism (v3.3). Despite evidence of significant secondary organic acid
formation, the explicit chemical box model largely underestimated their
formation in the oil sands plumes, accounting for 39, 46, 26, and 23 % of
the measured formic, acetic, acrylic, and propionic acids respectively and
with little contributions from biogenic VOC precursors. The model results,
together with an examination of the carbon mass balance between the organic
acids formed and the primary VOCs emitted from oil sands operations, suggest
the existence of significant missing secondary sources and precursor
emissions related to oil sands and/or an incomplete mechanistic and
quantitative understanding of how they are processed in the atmosphere.

From the atmospheric chemical process perspective, LMWOAs can be important
contributors to precipitation acidity and ionic balance, particularly in
remote areas (Khare et al., 1999; Stavrakou et al., 2012), and are
expected to become increasingly important as anthropogenic NOx and
SOx emissions continue to decrease. They are key participants in the
aqueous-phase chemistry of clouds and contribute to SOA formation through various reactions within the aqueous portion
of the particle phase (Carlton et al., 2007; Ervens et al., 2004; Lim et
al., 2010). Furthermore, since organic acids can be photochemical products,
they can also serve as indicators of atmospheric transformation processes.
Real-time and improved measurements of LMWOAs present a means of testing the
validity of current photochemical reaction mechanisms of VOCs from which the LMWOAs are ultimately derived.

From an
environmental health perspective, deposition of LMWOAs may have ecosystem
impacts as they have been shown to be toxic to various marine invertebrates
(Staples et al., 2000; Sverdrup et al., 2001), phytotoxic
(Himanen et al., 2012; Lynch, 1977), and interfere with
the uptake and mobilization of heavy metals by microbial communities in
soils (Song et al., 2016; Menezes-Blackburn et al., 2016). However,
studies on the human toxicity of LMWOAs are sparse and the results are unclear
(Rydzynski, 1997; Azuma et al., 2016).

The sources of gaseous organic acids are highly varied. Primary sources of
organic acids (LMWOAs in particular) include gasoline and diesel vehicle
exhaust emissions (Kawamura et al., 2000; Zervas et al., 2001a, b; Wentzell
et al., 2013; Crisp et al., 2014), biofuel
combustion, biological activity (i.e. direct vegetation emissions), soil
emissions, and biomass burning (Chebbi and Carlier, 1996). However, the largest source of LMWOAs is likely to be their formation from
the oxidation of VOCs (Paulot et al., 2011; Veres et al., 2011). The
most important oxidation pathway leading to their formation is likely to be
the gas-phase ozonolysis of alkenes, in which the further reaction of the
resulting stabilized biradicals with water (the dominant pathway under
tropospheric conditions) will form organic acids as a primary product
(Neeb et al., 1997). Other mechanistic routes include the OH-initiated oxidation of aromatic species (Praplan et al.,
2014), the decomposition of peroxyacetyl nitrate
(Surratt et al., 2009), and the OH
oxidation of vinyl alcohols (Andrews et al., 2012). Some volatile organic acids have also been shown to evolve to the
gas phase through heterogeneous aerosol reactions (Molina
et al., 2004) and/or aqueous chemical reactions involving glyoxal
(Carlton et al., 2007).
Despite the known primary and secondary sources of LMWOAs, the chemistry
forming LMWOAs from their precursors remains poorly understood. Incorporation
of known emissions and associated chemistry into global models has
repeatedly indicated missing sources, particularly with respect to formic
and acetic acids (Ito et al., 2007; Paulot et al., 2011; Von Kuhlmann et
al., 2003). While it has been stated that secondary sources of formic acid
and acetic acid from biogenic precursors (isoprene in particular) dominate
the global budget of these species, studies have demonstrated that large
inconsistencies exist between measurements and model predictions in the
Northern Hemisphere (Paulot et al.,
2011). Potential inconsistencies have not been examined for other LMWOAs.
However, given the limited measurements of other LMWOAs, as well as the
complex and likely incomplete mechanistic understanding of their formation,
larger discrepancies for other LMWOAs are expected. Large model–measurement
inconsistencies in the Northern Hemisphere, after having included biogenic
secondary sources, suggest the possibility of secondary formation from
anthropogenic VOC precursors, accounting for an increased fraction of the
global LMWOA budget. This points to the need for further measurements of
LMWOAs formed downwind of anthropogenic emissions.

One such anthropogenic source of LMWOA precursors is the oil and gas sector,
which has been expanding significantly in North America over the last
several decades and has significant VOC precursor emissions
(Gilman et al., 2013). Indeed, the formation of a
single LMWOA (formic acid) in an oil and gas region of the United States was studied recently
(Yuan et al., 2015)
during the winter season. The results of that study demonstrated that formic
acid was significantly under-predicted when using the current formic acid
photochemical mechanism, even after accounting for potential heterogeneous
and aqueous formation. Despite an entirely different VOC profile compared to
urban areas, secondary formic acid formation remained high in the region,
suggesting that anthropogenic VOC precursors from the oil and gas sector are
important contributors to formic acid formation.

The oil sands region of Alberta, Canada, represents another important oil- and
gas-producing region in North America with relatively large VOC
emissions (Li et al., 2017). It is estimated to contain up
to 1.7 trillion barrels of highly viscous oil mixed with sand (Alberta
Government, 2009) from which oil is recovered through unconventional surface
mining or in situ steam-assisted extraction. Rising oil sands oil production
has raised concerns over its environmental impacts, including those
associated with the deposition of toxic compounds, and the acidification of
nearby ecosystems via the deposition of SOx and NOx (Jung et
al., 2013; Kelly et al., 2009; Kirk et al., 2014). Recent evidence has also
indicated that the downwind transformation of oil sands gaseous precursors
(intermediate-volatility organic compounds (IVOCs) in particular) into SOA represents a
large PM input into the atmosphere
(Liggio et al., 2016). The
same photochemical processes that give rise to the observed SOA will also
lead to various other gas-phase photochemical products that include organic
acids. Given that the oil sands activities are surrounded by forests, the
oil sands represent an ideal location to study the primary emission and
secondary formation of gaseous organic acids from both industrial and
biogenic precursors in the absence of other confounding emissions.

In this work, we describe airborne measurements of gaseous LMWOA both
directly emitted from oil sands activities (i.e. primary) and formed via
secondary reactions of various oil-sands-emitted and/or biogenic hydrocarbon
precursors. Through the use of specifically tailored flight patterns and
top-down emissions retrieval algorithms, the total (and speciated) primary
emission and secondary production rates of LMWOA from the oil sands are
derived, from which the relative importance of primary and secondary organic
acids to the total organic acid budget in oil sands plumes is evaluated.
Through Lagrangian box modelling of successive oil sands plume intercepts,
the relative importance of biogenic and anthropogenic precursors within oil
sands plumes is examined, providing a means of evaluating our current
understanding of organic acid photochemical formation mechanisms for
selected LMWOA.

Molecular formulas and associated species names for organic acids
detected during aircraft measurements.

a Species name represents the molecule used for calibration, although not
all molecular formulas are for unique species. b Measured by PTR-ToF-MS;
see methods.

MethodsAircraft campaign

Airborne measurements of a variety of air pollutants aboard the National
Research Council Canada (NRC) Convair 580 aircraft were performed over
the Athabasca oil sands region of northern Alberta from 13 August to
7 September 2013 in support of the Joint Canada-Alberta Implementation Plan
on Oil Sands Monitoring (JOSM). Details regarding the overarching study
objectives, aircraft campaign implementation, and technical aspects have been
described previously (Gordon et al., 2015; Liggio et al., 2016). During
this study, 22 flights were conducted over the oil sands for a total of
approximately 84 h. To quantify
the total primary emissions for each facility, 13 of the flights were conducted by flying in the shape of a
four- or five-sided polygon, at multiple altitudes, resulting in 21 separate
virtual boxes around seven oil sands facilities. In addition, three flights (F7,
F19, F20) were conducted to study the photochemical transformation of
pollutants downwind of the oil sands (see the Supplement), including
the secondary formation of organic acids.

Gaseous organic acid measurements

Gaseous organic and inorganic acid measurements were conducted aboard the
aircraft with a high-resolution time-of-flight chemical ionization mass
spectrometer (HR-ToF-CIMS, Aerodyne Research Inc.). A detailed description
of the instrument and principles of operation has been given elsewhere
(Bertram et al., 2011; Lee et al., 2014) and in the Supplement. Briefly, the HR-ToF-CIMS used in this study was a
differentially pumped time-of-flight mass spectrometer configured to use
acetate ion as the reagent ion in the ionization of molecules of interest
(Veres et al., 2008; Brophy and Farmer, 2015) via the following reaction:
CH3COO-+HA→CH3COOH+A-,
where HA is the acid of interest and A- is the respective anion. Thus,
acids with a gas-phase acidity greater than that of acetic acid (Reaction R1) will
be ionized and subsequently sent into the mass spectrometer for detection.
There is also evidence that acetate dimers can be involved in the ionization
process and/or a number of clustering–declustering and deprotonation reactions
of acetate with organic acids (Brophy and Farmer, 2015). At the high
field strengths (i.e. strong declustering conditions) utilized during this
study (Yuan et al., 2016; Brophy and Farmer, 2015), it is expected that
such reactions ultimately also lead to a deprotonated acid ion (A- in
Reaction R1). Calibrations of organic acids were conducted both in the field and
post study using a liquid calibration unit (LCU, Ionimed Analytik GmbH), which
provided a stable gas stream of analytes by volatilizing aqueous organic
acid standards of known composition and concentration prepared from pure
compounds (> 99 %, Sigma Aldrich). A list of the quantified
organic acids is given in Table 1 and includes 18 acids spanning the
C1–C10 range.

The HR-ToF-CIMS data were processed using the Tofware software program
(Tofwerk AG, Switzerland), using an approach for mass calibration and high-resolution peak fitting that has been described previously (Brophy and
Farmer, 2015; Yuan et al., 2016), resulting in an overall conservative
uncertainty of ∼ 40 % in the quantified species
(Yuan et al., 2016). The accuracy of the
mass (m/z) calibration was approximately 5–7 ppm during the campaign. The
reagent acetate ion signal during this campaign was approximately
0.9–2.5×106 counts per second (cps), and was used to normalize the
analyte signals during post processing. The mass resolution of the
HR-ToF-CIMS during the study was approximately 3000 to 4000 for ions
spanning m/z=100 to greater than m/z=200. The quantified ions were
predominantly those whose signal dominated the nominal m/z space, minimally
perturbed by neighbouring shoulders and thus less affected by uncertainties
associated with HR peak fitting (Yuan et
al., 2016). Despite the high-mass-resolution, isomeric species (i.e. the
same exact mass) that are also acidic in nature (i.e. ionisable) cannot be
resolved from each other. For some organic acid species this is not
relevant, as no other acidic compounds with the same exact mass are likely
to exist in the atmosphere. Such species include formic acid, propionic
acid, acrylic acid and pyruvic acid. For all other species, the signal at
the exact mass was calibrated with the available surrogate standards listed
in Table 1. While this introduces a degree of uncertainty, calibrated
response factors across all species and m/z generally did not vary by more
than a factor of 2–3 and it is expected that organic acids with the same exact
mass will have response factors that vary considerably less than this.

A proton-transfer time-of-flight mass spectrometer (PTR-ToF-MS, Ionicon
AnalytiK) was used to measure acetic acid (which cannot be measured with the
HR-ToF-CIMS) in real time during each flight. The PTR-ToF-MS is a soft
ionization technique that detects VOCs with a proton affinity greater than
that of water. This includes species such as unsaturated hydrocarbons,
aromatics, and various oxygenated compounds. Details of the PTR-TOF-MS
technique and application have been described previously (Graus et al.,
2010; De Gouw and Warneke, 2007; Li et al., 2017). The deployment of the
PTR-ToF-MS on the aircraft during this study, its calibration, and data
processing have been described elsewhere (Li et al., 2017). Acetic acid via the PTR-ToF-MS was calibrated using a method described
previously (De Gouw et al., 2003; Zhao and Zhang, 2004) by calculating
a response relative to a known and calibrated reference compound (toluene in
this case). The response factor for acetic acid (RAA) is calculated as
RAA=kAATAARTolkTolTTol,
where k refers to the kinetic rate constant for the reaction of acetic acid
or toluene with H3O+, T is the experimentally determined ion
transmission efficiencies (C2H5O2+, m/z=61 and
C7H9+, m/z=93), and RTol is the experimentally derived
response factor for toluene (using a standard gas cylinder). The response
factor for acetic acid calculated in this manner was within 10 % of that
derived using a permeation device during previous studies.

Other supporting measurements

A detailed description of the meteorological variables, aircraft state
parameters and a full list of gas–particle measurements is provided elsewhere
(Gordon et al., 2015; Liggio et al., 2016) (http://jointoilsandsmonitoring.ca/default.asp?lang=En&n=A743E65D-1). The current work makes use of a subset
of these measurements. These include refractory black carbon (BC) measurements via a single-particle soot photometer
(SP2,
Droplet Measurement Technologies, Boulder, CO, USA), VOC canister sampling
followed by offline analysis and nitrogen oxides (NO and NO2), and
ozone measurements (TECO 42i-TL and 49i respectively, Thermo Fisher
Scientific, Waltham, MA, USA). Details with respect to supporting
measurements are provided in the Supplement.

Top-down emission rate retrieval algorithm (TERRA)

A top-down emission rate algorithm (TERRA) designed to estimate pollutant transfer rates through
virtual boxes or screens from aircraft measurements was used to derive the
primary and secondary emission and production rates from the oil sands flights
(Gordon et al., 2015 and Supplement). Briefly, primary emissions are derived
utilizing box-like aircraft flight patterns surrounding each of the main
surface-mining facilities, pollutant measurements at high time resolution,
and wind speed, direction, temperature, and pressure data. Organic acid
measurements, at high time resolution, can be used directly in TERRA for
estimating their primary emissions; however, they are also secondary
products,
which may form via chemistry in the short distance and time between the source
area and the virtual box wall. Hence, direct use of TERRA for estimating
primary organic acid emissions may result in their overestimation.
Alternatively, organic acid primary emissions are estimated after
normalization to BC in the source area and scaling by the BC emissions via
an approach that is described in section “Primary LMWOA Emission Rate Estimates”.

Secondary formation rates of organic acids were also derived with an
extended version of TERRA using Lagrangian transformation flights (F19,
F20). The extended TERRA quantifies the mass transfer rate of pollutants
(kilograms per hour) across the virtual screens of transformation flights, in the same
manner as for virtual box-type flights, and has been described in detail for
SOA
(Liggio et al., 2016) and in
the Supplement. Briefly, for the transformation flights, TERRA
is applied to single screens created by stacking horizontal legs of flight
tracks at multiple altitudes and spirals. Concentration data are mapped to
the screens and interpolated using a simple kriging function. Pollutant
concentrations are also extrapolated below the lowest flight altitude
(150 m a.g.l.) based on the assumption of a well-mixed layer below the lowest flight
track. It has been demonstrated that this extrapolation is the main source
of uncertainty in TERRA, resulting in overall uncertainty in the derived
pollutant transfer rates of approximately 20 % (Gordon et al., 2015;
Liggio et al., 2016).

Emissions box flight during Flight 18 on 3 September 2013
(Syncrude – ML) as outputted from TERRA for a typical measured organic acid
(C5H8O2). Data have been kriged to produce a continuous
concentration surface around various oil sands operation sources. Dashed black
lines represent approximate locations of the flight tracks on one box
wall.

Box modelling

A photochemical box model (AtChem Online, atchem.leeds.ac.uk) coupled with
the Master Chemical Mechanism (MCM v3.3, University of Leeds, http://mcm.leeds.ac.uk/MCM/) (Jenkin et al.,
2012) was used to simulate the individual organic acid (and total) formation
and evolution during transformation Flight 19, as well as ozone and free
radical production and other photochemical products. The specific data
points within the plume to simulate with the model were selected to be those
that were very close to being truly Lagrangian in nature as determined from
the wind speed, wind direction, and flying time. The successive plume
intercepts modelled here, in which the same air
parcel was sampled typically within 1 min of its arrival time based on
the wind, are depicted in Fig. S1 in the Supplement. The model consisted of an explicit mechanism for 4578 species in
18 045 reactions. Further details with respect to modifications to the MCM
for organic acids are given in the Supplement and in Yuan et
al. (2015). Photochemical rate constants were calculated using the Tropospheric
Ultraviolet and Visible (TUV) radiation model (https://www2.acom.ucar.edu/modeling/tropospheric-ultraviolet-and-visible-tuv-radiation-model)
and were constrained along the flight path. Relative humidity, temperature,
and pressure were also constrained with measurement data along the flight
path. CO and biogenic VOCs (isoprene, α-pinene, and β-pinene) were constrained via measurements along the entire flight path of
the aircraft to account for biogenic emissions between screens. Measured
cycloalkanes were lumped into cyclohexane (the only cycloalkane in MCM
v3.3), and any other VOCs not present in the MCM were lumped into a VOC with
similar reactivity in the MCM. The model was initialized and constrained
using the measurements of VOCs (including organic acids), NOx, CO, O3,
SO2, and other parameters at the first screen. The model was run for 3 h to correspond with the flight time and with a time step of 1 min.
BC measurements were used to derive first-order dilution rate
constants and applied to all species to account for ongoing dilution.
Species within the plume were diluted at every time step with air outside
the plume whenever background measurements of the acids were available;
otherwise, clean air was used as background.

(a) Time series of measured propionic acid and BC during Flight 18
(Syncrude – ML). Multiple plume intercepts at the virtual box wall are
shown. (b) Correlation between propionic acid and BC during F18. Black data
points represent the means, and blue boxes and whiskers are the 25th
to 75th percentiles and 90th and 10th percentiles
respectively. (c) Correlations between various measured organic acids during
F18. Error bars represent the 25th to 75th percentiles of the
data.

Results and discussionPrimary LMWOA emission sources

Primary emissions of various pollutants from specific surface-mining oil
sands facilities were estimated by flying virtual boxes around operations
followed by subsequent analysis using TERRA (Gordon et al., 2015; Liggio et
al., 2016; Li et al., 2017). The specific oil sands facilities that were
evaluated are shown in Table S1 in the Supplement with corresponding flight numbers and
include Syncrude, Suncor, Canadian Natural Resources Limited (CNRL), Shell, and Imperial Oil. The geographical
locations of these operations in relation to the entire oil sands region are
provided elsewhere (Li et al., 2017). The results of a
typical emission flight (F18, Table S1) are shown in Fig. 1 for a C5
organic acid, after having applied the simple kriging method to derive a
continuous concentration surface. The enhancement in the concentration of
this C5 acid on the downwind box wall (Fig. 1) clearly demonstrates
that this species is associated with oil sands activities from this
particular facility. Similar enhancements for all other measured organic
acids at this facility (and others) and during other flights of Table S1
were also observed. While this organic acid and most others are clearly
derived from an oil sands source, generally, surface-mining operations
consist of a variety of potential pollutant sources as outlined in Fig. 1,
including open pit mines, tailing ponds, and processing plants. These sources
are all often in close proximity to each other. However, the chemical nature,
location, and extent of individual oil sands plumes within facilities,
relative to the prevailing winds, can often be used to determine specific
sources. For example, while mines and tailing ponds will lead primarily to
surface emissions, processing plant emissions may be mostly from elevated
stacks, particularly for SO2 (Gordon et al.,
2015). In the case of LMWOA, there is a primary emission source attributed to
the use of off-road heavy duty diesel vehicles within the open pit mines, as
suggested by several observations described below. Firstly, the prevailing
winds and positioning of the concentration enhancements at the box wall of
each flight indicate that the open pit mines are the most likely source of
the LMWOA emissions (for example in Fig. 1). Within the mines, emissions can arise from the vehicles within the mine, or via volatilization
from the mine face itself. Emissions of LMWOA via volatilization is highly
unlikely as the oil sands material that is mined is extremely rich in hydrocarbons
and deficient in oxygenates and would nonetheless be observed as a more
spatially homogenous source (which was not evident). The same is also true
for the potential heterogeneous oxidation of the oil sands ore leading to
LMWOA. Secondly, LMWOAs during box flights were consistently correlated with
BC and not correlated with other species such as SO2. An
example of this relationship is given in Fig. 2a for propionic acid during
F18. Figure 2a demonstrates that there is a high degree of similarity
between the time series of propionic acid and BC, particularly at the plume
intercepts on the virtual box wall. Since the majority of BC
emissions within oil sands operations arise from the heavy hauler trucks used
in the open pit mining (Wang et al., 2016), the
observed LMWOA : BC correlation is consistent with an emission from the large
heavy-hauler diesel trucks within the mine and with the known emissions of
various LMWOAs from diesel fuel combustion (Kawamura et al., 2000; Wentzell
et al., 2013; Crisp et al., 2014; Zervas et al., 2001b). Finally, flights
directly over the mines indicate that there are LMWOA emissions at that point
specifically (and not elsewhere), and further secondary formation of LMWOA
moving away from the primary emission point (see section “Primary LMWOA emission rate estimates”). The
relationships and observations above were also evident for other emission flights,
and for all other LMWOAs measured. The correlation between LMWOA and BC is
subsequently used to derive individual and total LMWOA primary emission rates
from various oil sands facilities (after accounting for potential secondary
formation), as described further below.

Concentration of BC during emission Flight 18 (Syncrude – ML),
showing horizontal transects A–C within the box and closest to the
mining emission source (red box). Time in brackets represents the
approximate time between horizontal transects or from the approximate centre
of the mine to the closest transect. The time is calculated based upon the
average wind speed during this portion of the flight. The range in values is
based upon differences in time calculated from the northernmost and southernmost legs of transects A–C.

For conserved and chemically unreactive species, the virtual box flights can
be used directly in TERRA to estimate emissions (kilograms per hour) via a mass balance
approach. However, the distance between oil sands sources and any given box
wall in an emission flight can range from 10 to 15 km. With the average wind
speeds during these flights, such a distance corresponds to approximately 10–60 min in transport time. During this time, it is possible that
photochemical reactions that could increase or decrease the
concentration of a given pollutant at the box wall occur, and hence affect the
final emission rate calculated by TERRA. This has been accounted for in the
case of VOCs by estimating the rates of oxidation for specific hydrocarbons
during travel to the exit of the virtual box, using known rate constants for
the reaction of VOC with OH and O3 (i.e. kOH and KO3)
(Li et al., 2017). This has been shown in most instances
to result in small corrections to the TERRA-derived emission rates for the
hydrocarbons (Li et al., 2017). The degradation of
organic acids during transport to the box wall is expected to be slow since
their OH and O3 rate constants are generally small. However, they are
more importantly photochemical products of VOC oxidation.
Attempting to correct for the contribution of their photochemical formation
prior to input into TERRA is not feasible as it requires detailed knowledge
of all oxidation mechanisms leading to LMWOAs and their associated yields.
Such information is not available and would nonetheless carry a high degree
of uncertainty.

Alternatively, BC is used as a normalizing tracer to estimate facility
emission rates. As noted above, the time series of LMWOA and BC are similar
(e.g. Fig. 2a), suggesting a common incomplete combustion source from the
large mining trucks. The correlation between propionic acid and BC for F18
(for example) is shown in Fig. 2b and indicates that while the two
species are correlated, a significant spread in the data exists (denoted by
the 25th–75th percentiles), likely caused by a degree of
photochemical propionic acid formation while BC is conserved. This is also
reflected in the observation that other LMWOAs are more significantly
correlated with each other (Fig. 2c) than they are with BC (i.e. LMWOAs are
all more similarly formed and lost relative to BC). These observations
further imply that the formation of LMWOAs will be reflected in the
evolution of the background-subtracted LMWOA to BC ratio (ΔLMWOA /ΔBC) between the emission source and virtual box wall. In
this case, the ΔLMWOA /ΔBC at the source (i.e. the emission
ratio) can be used to estimate facility primary LMWOA emissions
(ELMWOA, kg h-1), when the corresponding BC emissions EBC from the virtual
boxes over the individual facilities have been determined using TERRA
(Cheng et al., 2017), according to Eq. (2):
ELMWOA=ΔLMWOAΔBCSource×EBC.
LMWOA emission ratios ((ΔLMWOA /ΔBC)source) were derived
from flights where horizontal transects between the centre of the open pit
mines and the exiting box walls were flown. These flights included F17, F18,
and F21 (spanning four facilities) and are depicted in Figs. 3, S2, and S3.
These figures indicate the horizontal extent of the BC plumes from the
various mines and the approximate times and distances from the approximate
centre of a given mine to the various horizontal transects. At the closest
transects to the mine centres, the time from emission was estimated to range
from approximately 0.3 to 6 min based on the average wind speeds during
these flights. This is a time during which photochemical production of the LMWOA is
expected to be minimal and the ΔLMWOA /ΔBC ratio should
approach the true emission ratio, unlike that expected during the longer
transport time from emission to the box wall (10–60 min; see Li et al.,
2016) The evolution of ΔLMWOA /ΔBC for the four depicted oil
sands operations, at the various horizontal transects of Figs. 3, S2, and
S3 (e.g. A, B, C, A1, B1, and C1), are shown in Fig. 4 for a
selected organic acid. The boxes in these figures represent the 25th to
75th percentiles of the individual ΔLMWOA /ΔBC ratio
values within the plume only, as defined spatially by the BC (which is
approximately zero outside of the plume), and at approximately the same
altitude. These figures demonstrate a degree of photochemical LMWOA
formation when moving from the emission sources to the virtual box walls.
Hence, the emission ratios for the various species are considered to be the
ratio in the yellow highlighted regions of Fig. 4 (i.e. the closest
approach to the mine source – A, A1, and A2).

Photochemical formation of organic acids from co-emitted hydrocarbons within
the mines is not expected to significantly contribute to the derived
emissions ratios in the 0.3 to 6 min travel time. This is particularly
likely for the transects closest to the mine sources, as very high
co-emitted NO at the source titrates O3 to levels below 15 ppbv (Fig. S4) and is likely to effectively suppress active photochemistry at the
source via OH radicals. Furthermore, most LMWOAs are likely later generation
products of VOC oxidation and not likely formed to a great extent in such a
short time. Nonetheless, a small secondary LMWOA contribution from
co-emitted hydrocarbons to the emission ratios here cannot be entirely ruled
out.

The emission ratios (ΔLMWOA /ΔBC) derived for individual
LMWOA species for the four facilities shown in Fig. 4 are presented in Fig. S5, in which the error bars represent the 25th to 75th percentiles
of the computed ratio data. Generally, the profiles of emission ratios in
Fig. S5 are relatively similar to each other, regardless of the facility
(means within 15–70 % for individual species). However, the
differences between facilities may be attributed to differing emissions
control systems between recent and older model mining vehicles. The largest
emission ratios are associated with formic and acetic acids, ranging from
412 to 800 pptv/µg m-3, followed by propionic, butyric, and
pentanoic acids in the range of 114 to 205 pptv/µg m-3. Other
LMWOAs have smaller but non-negligible emission ratios. The corresponding
facility LMWOA emission rates (kilograms per hour) derived with Eq. (2) (a primary
emission rate estimate) are shown in Fig. S6. Where individual emission
ratios for a given facility were not available, the mean of the emission
ratios for each species is used to compute the emission rate (i.e. for Suncor
– MS and Imperial – KL). Accordingly, the largest primary emission rates
are observed for formic and acetic acids (65 and 129 kg h-1, summed
across facilities), followed by propionic, butyric, and pentanoic acids at
52, 54, and 40 kg h-1 respectively. This corresponds to the fractional
contributions to the total measured LMWOA mass emission rate shown in Fig. S7a, in which formic and acetic acids alone account for ∼ 43 %
of the primary emissions.

From a facility standpoint, speciated and total measured primary emissions
of LMWOA during this study were variable (Fig. S6), with total LMWOA
emissions for the Suncor-MS, Syncrude-ML, Syncrude-AU, Shell-MR/JP, CNRL-HOR,
and Imperial-KL facilities estimated be 162 ± 22, 108 ± 15,
45 ± 6, 56 ± 8, 60 ± 8, and 19 ± 3 kg h-1
respectively. The total primary LMWOA emission rates should also then be
somewhat proportional to oil sands oil production, given that LMWOAs are
associated with diesel exhaust during mining activities. Since BC emission
rates were found to be linearly correlated with the quantity of oil sands
mined (Cheng et al., 2017), LMWOA emission rates derived
using BC emission rates as the tracer are therefore similarly correlated
with the oil sands mined. This correlation suggests that primary LMWOA
emissions may also be expected to track increases or decreases in oil sands
productivity.

The sum of the measured emission rates for LMWOA (i.e. total primary) is
shown in Fig. 5, as well as the measured primary emissions of total and
oxygenated VOCs (Li et al., 2017). In total, the oil
sands are estimated to emit approximately 12 t day-1 of primary
LMWOAs (see the Supplement). Relative to the total VOC emitted
simultaneously (≈ 214 t day-1), the emission of LMWOA is
small, representing an increase to the total VOC emissions of
less than 6 %. However, of the total VOC emissions, oxygenated VOCs have
been estimated to account for < 10 % (≈ 20 t day-1) (Li et al., 2017) (Fig. 5) and were comprised
mainly of methanol, formaldehyde, and acetone. Hence, the LMWOA emissions
here represent an increase of up to 60 % to the oxygenated VOC mass
emissions, which have previously been unaccounted for and for which there
are currently no regulatory reporting requirements.

The primary LMWOA emissions from the oil sands are not easily placed in
context, as measurements of emission rates from various other anthropogenic
sources are generally not available. On a global scale, primary sources of
LMWOAs have been estimated to be very small relative to their photochemical
production from a variety of precursor gases
(Paulot et al., 2011). This
indicates that while possibly important on a regional scale, the primary
emitted LMWOAs associated with the oil sands are not expected to contribute
significantly to the overall organic acid atmospheric burden both in the
Canadian context and/or globally. Given the large emissions of VOCs from oil
sands operations (Li et al., 2017), it is likely that
secondary formation of LMWOAs from precursors derived from oil sands is more
significant than their primary emissions, a subject to be further explored
below.

The secondary formation rates of LMWOAs are estimated using a modified
version of TERRA with transformation Flights 19 and 20 (see methods, Sect. 2.5). An example of the resultant screens from which secondary formation
rates are derived is given in Fig. 6 for C5H8O2. Using BC
measurements to define the spatial dimensions of the oil sands mine plume (red
boxes, Fig. 6), the formation rates for any specific LMWOA is given as the
difference in transfer rates between screens (1 to 4). Accordingly, for
C5H8O2, approximately 219 ± 43 kg h-1 is formed
downwind of the oil sands in the 3 h between screens 1 and 4.
Similarly, the formation rates for all measured LMWOAs between screens 1 and
4 during F19 are shown in Fig. 7a, along with the sum of the estimated
primary LMWOA emissions of Fig. S6 (summed over all facilities). From
Fig. 7a it is evident that primary emissions of LMWOAs are negligible
compared to those formed via oxidation of precursors; the sum of primary
emission rates (across all facilities) is on average 30 times lower than
the transfer rates through screen 4 (Fig. 7a). Consequently, the total
speciated secondary formation rates (between the source area and screen 4,
i.e. ≈ 4 h) are given as the transfer rates at screen 4 (kilograms per hour) after having subtracted the small primary emission
contributions; these are shown as the green bars in Fig. 7a. During this 4
hr midday time period, individual secondary production rates of LMWOAs
ranged from ≈ 20 to 6700 kg h-1, dominated by acetic (≈ 6700 kg h-1) and formic acids (≈ 3200 kg h-1), with all
other species in the ≈ 20–500 kg h-1 range. On a percent
basis, formic and acetic acids together account for ∼ 70 %
of the total estimated secondary LMWOA production rate downwind of the oil
sands, with all other organic acid species generally contributing less than
3 % each to the total (Fig. S7b). This is in contrast to the estimated
primary emission rates from the oil sands, in which formic and acetic acids
accounted for ∼ 43 % of the total measured primary LMWOA
emission rate, with several other species accounting for up to 12 %
(Fig. S7a). Such relative differences between LMWOA secondary production
and primary emission rate profiles is expected since photochemical
formation of LMWOA is likely to occur at rates and via formation mechanisms
that will differ from those of primary combustion sources. Furthermore,
formic and acetic acids being the lowest MW acids are likely to have many
more precursors than other LMWOAs, implying that their relative proportions
to the total LMWOA should increase with increasing photochemical processing.

TERRA-derived concentration screens for F19 using
C5H8O2 as a relevant LMWOA example. The LMWOA transfer rate
difference between screens represents the secondary production rate of a
given species (yellow text, 219 ± 43 kg h-1 for screens 1–4). The overall rate from the oil sands source region is the integrated
LMWOA transfer rate through screen 4 after subtracting a small primary
emission rate (336 ± 67 kg h-1; see text).

The hourly formation rates of Fig. 7a are further scaled to 1
photochemical day (i.e. t day-1) as previously described in detail
(Liggio et al., 2016). While
oil sands operations are active 24 h per day, a simple scaling up by
multiplying the hourly rates by 24 may add significant uncertainty, as
photochemistry does not occur uniformly across these hours. Alternatively,
the hourly LMWOA production rate (for each hour) was scaled by the time-integrated OH radical concentration, which was modelled with a box model
(Liggio et al., 2016). The
daily LMWOA production rate is then the sum of the scaled hourly production
rates and is shown in Fig. 7b for Flights 19 and 20. Typically, the
scaled daily secondary production rates were ∼ 54 % of the
daily rate derived using a simple 24 h multiplier; 184 ± 37 and 173 ± 34 t day-1 for F19 and F20 respectively.
This estimation neglects the dry deposition of LMWOA, which may be
significant. Accounting for the potential deposition of these species (see
the Supplement) increases the estimated total secondary LMWOA
formation rates even further to 288 ± 58 and 226 ± 45 t day-1. The total LMWOA secondary production rate
(excluding deposition as a lower limit) is also compared to the total
measured hydrocarbon VOC emissions from oil sands operations in Fig. 7b.
On a total mass basis (tonnes per day) LMWOA production is
almost as large as the total VOC primary emissions (∼ 214 t day-1) (Li et al., 2017). At least two oxygen
atoms are added during oxidation (to form acids), and hence a carbon mass
comparison is more relevant and is also shown in Fig. 7b.

This comparison indicates that up to 50 % of the organic carbon mass
emitted is transformed into organic acids (i.e. an effective yield of
approximately 50 %) in one photochemical day. Conversely, typical yields
for LMWOAs from VOCs in smog chamber experiments are less than 10–15 % and
often much lower (Paulot et al., 2009; Neeb et al., 1997; Butkovskaya et
al., 2006; Berndt and Böge, 2001; Yuan et al., 2015). This suggests
that the total primary VOC emissions have been significantly underestimated
and that numerous other hydrocarbon species have not been measured, despite
having quantified > 150 individual VOC species. This is
consistent with the pool of semivolatile and intermediate-volatility compounds
(SVOC / IVOC) expected to be emitted (but not measured), which were responsible
for the observed SOA in oil sands plumes
(Liggio et al., 2016). The
ability of semivolatile and intermediate-volatility hydrocarbons to form LMWOAs is
unknown. However, IVOC / total VOC emission ratios for the oil sands have been
suggested to be large (Liggio
et al., 2016), indicating that yields of LMWOAs from these species can have
a significant impact on the total LMWOA production rate. However, from a
carbon mass balance perspective, even LMWOA yields of ≈ 10 % from
IVOCs would require that the total emitted carbon in the form of IVOCs
exceeds the measured VOC carbon emissions (≈ 175 t C day-1, Fig. 7b) by at least a factor of 5 in order to produce the
≈ 75 t C day-1 of LMWOA estimated in Fig. 7b. While
there is no evidence to the contrary, such a large IVOC contribution to
total oil sands emissions may be unlikely. This may suggest that LMWOA yields
from IVOCs may be larger than typical VOCs. In this regard, recent evidence
has suggested that molecular fragmentation during the oxidation of IVOCs,
specifically, can dominate over functionalization (and hence SOA formation)
in significantly less time than a photochemical day
(Lambe et al., 2012). Molecular
fragmentation to smaller species such as LMWOAs in the highly
photochemically active plumes encountered here
(Liggio et al., 2016) could
significantly increase LMWOA yields relative to laboratory yields from
experiments, which are typically performed at moderate OH and for less time than a
photochemical day (Paulot et al., 2009; Praplan et al., 2014). Biogenic
hydrocarbons present along the flight track in F19 and F20 also have the
potential to contribute to the observed LMWOA production rate and have not
been included in the estimates of total oil sands VOC emissions (Fig. 7b) (Li et al., 2017). However, Lagrangian box modelling
of F19 indicates that the biogenic contribution to several LMWOAs is
relatively small as described in Sect. 3.2.3.

(a) Secondary LMWOA production rates (kilograms per hour) derived between
screens 1 and 4 of F19 using the TERRA algorithm, and at screen 4 after
removing the primary LMWOA contribution (green bars). The primary LMWOA
emission rates are also shown (grey bars). (b) Total LMWOA secondary
production rates (F19 and F20 neglecting deposition) extrapolated to 1
photochemical day compared to total reported primary VOC emissions and
carbon associated with these VOC emissions (Li et al., 2017). Data for
organic acid and VOC carbon are stacked.

The current LMWOA secondary production rates represent the first such
estimates downwind of a large-scale industrial facility. Indeed,
measurements of LMWOA secondary production rates from any source are
generally not available, and hence placing the LMWOA production from oil
sands precursors in context with that of other sources is difficult.
Available estimates of secondary anthropogenic production rates of LMWOAs
have been limited to formic and acetic acids, and only in a global context
(Ito et al., 2007; Paulot et al., 2011). Nonetheless, the most recent
estimates indicate that the sum of secondary anthropogenic and biomass
burning sources of formic acid and acetic acid contribute approximately 6.3
and 1.3 Mt per year (Mt yr-1) respectively to the global
budget (Paulot et al., 2011). Crudely downscaling (365 days yr-1), this results in a global daily anthropogenic
(plus biomass burning) production rate of approximately of 2.1×104 t day-1 (sum of formic acid and acetic acid). Hence, the combined
formic acid and acetic acid secondary production rates observed in oil sands
plumes (∼ 129 t day-1) likely contribute < 1 % to the global secondary anthropogenic and biomass burning budget. While
only qualitative, this comparison suggests that the oil sands are not a
major secondary anthropogenic source of formic acid and acetic acid
globally.

The impact of the oil sands on a smaller scale (e.g. regional), as a
photochemical producer of formic acid and acetic acid (and other organic
acids), is not clear since comparative data are not available. However, we
note that the total LMWOA formed within a photochemical day of the oil sands
in this study (up to ≈ 184 or ≈ 288 t day-1 if accounting for deposition) is comparable to the total SO2
previously reported to be emitted from the oil sands (∼ 200–300 t day-1) (Hazewinkel et al., 2008; Jung et al., 2011).
The strong acidity associated with sulfur deposition is likely to dominate
downwind ecosystem effects. However, the impact of weak acidity on ecosystem
acidification has been previously considered
(Vet et al., 2014) and
may be particularly important in remote areas (Stavrakou et al., 2012;
Keene, 1983). The impact of weak acidity downwind of the oil sands
specifically is not clear. Critical load exceedances for highly sensitive
aquatic systems in northern Alberta have mainly been assessed from the
perspective of sulfur and nitrogen (Cathcart et al., 2016; Whitfield et
al., 2016). While the impact of the large amount of weak organic acidity
formed downwind of the oil sands has not been evaluated, it may have a
relevant impact on ecosystem acidification in highly sensitive systems that
are approaching their respective sulfur and/or nitrogen critical loads.
These results warrant a further investigation of the potential impact of
this LMWOA emission and formation in this context.

Modelling secondary LMWOA formation in oil sands plumes

While organic acid secondary production rates are large during F19 and F20
relative to oil sands hydrocarbon primary emissions (Fig. 7), the specific
precursor species leading to these observed acids were not clear, including
the impact of biogenic volatile organic compound (BVOC) oxidation. However, the Lagrangian nature of F19 allows a
comprehensive box model evaluation of the most recent photochemical
mechanisms leading to LMWOAs and an estimate of precursor contributions.
Flight 19 was modelled for 3 h, beginning at the first screen and ending
at the fourth screen, utilizing measurements of all VOCs and inorganic gases
at screen 1 as the initial conditions. The most recent Master Chemical
Mechanism v3.3, with further improvements for formic acid and acetic acid
mechanisms, was used to simulate the secondary chemistry. A detailed
description of the box modelling approach is given in methods (Sect. 2.5).
As demonstrated in Fig. S8, the box model effectively simulates the
evolution of some known organic and inorganic gases, including the oxidative
loss of alkenes, alkanes, and aromatics (and others); the cycling of NOx, and
ozone formation; this provides confidence in the interpretation of other
aspects of the model output. The comparison between measured and modelled
LMWOAs during F19 is shown in Fig. 8. The model output is compared with
four specific organic acids (formic, acetic, acrylic, and propionic) as these
species are certain to be free of isomeric organic acid interferences in the
HR-ToF-CIMS measurements. Figure 8 indicates that the formation of these
four species is poorly simulated by the box model. The model and measurement
time series diverge quickly (< 30 min), with the model output
dominated by dilution (causing the downward trend) while the observed acids
increase over 3 h, suggesting that the formation rate of these species
is sufficient to overcome the effects of dilution and deposition. After 3 h, the model under-predicts the concentrations of formic, acetic,
acrylic, and propionic acids in the plume by factors of 2.6, 2.2, 3.9, and 4.4
respectively. Similarly, the total measured LMWOAs are also under-predicted
(Fig. 8) by a factor of 2.9, where 27 modelled organic acids account for
99.8 % of the LMWOA mass as compared to the 18 measured species.
Accounting for the depositional losses for these species in the model will
increase the model–measurement discrepancy even further. Such poor
model–measurement agreement is again consistent with significant unaccounted-for VOC / IVOC emissions, which lead to various LMWOAs, from the oil sands, as
was suggested by the unrealistically high effective organic acid yield of
50 % in Fig. 7b (Sect. 3.2.1). This is particularly true for formic
acid, whose formation in the MCM has been recently updated
(Yuan et al., 2015)
and yet remains poorly simulated.

The model output also allows for a more detailed examination of the
precursors responsible for a portion of the organic acids observed in oil
sands plumes. The relative contribution of various precursor types to the
modelled formic, acetic, acrylic, and propionic acids after 3 h during
F19 (screen 4) is shown in Fig. 9. These contributions by the precursors
indicate that the oxidation of biogenic emissions along the flight track
were largely not major contributors to the observed levels of these species
relative to the oxidation of oil sands emissions. Formic and acetic acids
have received significant attention recently with respect to their
photochemical production mechanisms from biogenic species and have been
updated in the current MCM used here (see methods). Despite this update, the
combined oxidation of isoprene and monoterpenes accounted for approximately
18 and 33 % of the formic and acetic acid produced after 3 h, with
measured aromatics, alkenes, and alkanes accounting for ∼ 2–11 % each. The biogenic contributions to acrylic and propionic acids are
even smaller (Fig. 9), with isoprene oxidation contributing
∼ 3 % to acrylic acid formation and propionic acid having no
biogenic precursor contribution. The completeness and/or validity of the MCM with
respect to acrylic and propionic acids is unclear; it has not been recently
updated, as few experimental studies exist to validate MCM yields of these
species from various precursors. However, yields from biogenic VOCs for
these two species are not likely to exceed those of formic acid and acetic
acid, making the contribution to acrylic and propionic acids small.

Comparison between specific measured and modelled organic acid
species (formic, acetic, acrylic, and propionic) for F19. Total measured and
modelled LMWOAs are shown as grey and purple shaded regions. Superscript “a”: total
modelled LMWOA concentration represents the sum of 27 species, accounting
for ∼ 99.8 % of modelled LMWOA mass.

Relative contribution of various precursor hydrocarbon types in
the photochemical box model to the modelled secondary production rate after
3 h of evolution during F19, for formic, acetic, acrylic, and propionic
acids.

Undoubtedly, the largest contribution to these four LMWOAs is unaccounted
for and labelled as missing in Fig. 9. Such missing precursor sources
account for ∼ 54–77 % of production of these species
after 3 h of processing. Although other processes such as aqueous and
heterogeneous chemistry can lead to LMWOAs and are not included in the MCM
mechanism here, their contribution to formic acid formation was estimated to
be < 5 %
(Yuan et al., 2015). Similarly, fog events and air–snow exchange have no contribution here (i.e.
fog and snow were not present). For formic acid, this missing contribution
is similar in magnitude to that observed in another oil- and gas-producing
region (Yuan et al.,
2015). The source of the missing contributions can be a result of several
factors: incorrect yields of LMWOA from species currently in the MCM,
unknown mechanistic pathways leading to LMWOAs from various MCM precursors,
and unmeasured hydrocarbon precursors that hence were not included in the
MCM. The first factor appears to be minor, and in the case of formic acid
and acetic acid the updated yields from various hydrocarbons in the MCM are
considered upper limits
(Yuan et al., 2015)
and further updates to yield data will likely not result in significantly
more formic acid and acetic acid contributions. For the second factor, LMWOA
yields from alkanes are not included in the current MCM (with the exception
of cyclic alkanes), although limited evidence suggests that some LMWOAs may be
indirectly formed via their oxidation (Zhang et
al., 2014), but they are insufficient to account for a significant portion of the
missing mass in Fig. 9. Given the unrealistically high LMWOA effective
yields estimated here (up to 50 %) and the small contribution of BVOC
oxidation in these plumes, the third factor is the most likely reason for
the missing contributors; their absence may be entirely due to the oxidation of oil-sands-related emissions that were not measured. Furthermore, given the
known presence of a large amount of low-volatility oil sands hydrocarbons
in these plumes that give rise to SOA
(Liggio et al., 2016), it is
possible that the large fraction of the missing term will be IVOC,
whose photochemical mechanisms have yet to be elucidated, in nature but as noted
above are susceptible to significant fragmentation
(Lambe et al., 2012), potentially
leading to LMWOA.

Conclusions

Measurements of 18 gas-phase organic acids have been made in the oil sands
region of Alberta, Canada, for the first time, indicating that they are
emitted from primary sources and formed via secondary chemistry in oil sands
plumes. The main primary source of these acids is demonstrated to be
combustion within open pit mines, leading to primary LMWOA emissions of up to
12 t day-1. This suggests that primary oxygenated emissions from oil
sands activities are likely larger than previously considered, with organic
acids comprising a large fraction of these emissions.

Despite the potential important contribution of primary LMWOA emissions to
the oil sands emissions inventory, secondary formation rates in oil sands
plumes were found to be dominant over primary emission rates by more than one
order of magnitude; secondary formation rates within 1 photochemical day of
the oil sands were in excess of 180 t day-1 and were the first
estimates of their kind from any industrial or urban–suburban source. Consequently, up
to 50 % of the carbon emitted was transformed to organic acids within 1
photochemical day. This is an unusually high effective yield, which suggests the
presence unknown–unmeasured hydrocarbons that are capable of producing
LMWOAs upon oxidation with significant yields. The Lagrangian nature of
several flights during this study also provided a unique opportunity to
examine the ability of current photochemical mechanisms to reproduce LMWOA
observations. Subsequent box modelling of the evolution of several organic
acids over the course of 3 h significantly under-predicts the
concentration of simple organic acids. A missing source accounts for the
majority of the species measured (54–77 %) but is not related to the
oxidation of biogenic species. The poor model predictive ability and the
lack of biogenic contribution suggests missing oil sands precursors that
are likely to be of intermediate volatility. These results suggest that
further work is required to understand the nature of these missing
precursors, to elucidate their photochemical pathways leading to LMWOA and
other products, and to narrow the model–measurement gap. Finally, the impact of
the weak acid deposition to sensitive ecosystems and contribution to overall
critical load exceedances is not clear but likely warrants further
investigation.

The Supplement related to this article is available online at https://doi.org/10.5194/acp-17-8411-2017-supplement.

The authors declare that they have no conflict of interest.

Acknowledgements

We thank the National Research Council (NRC) of Canada flight crew of the
Convair 580, the technical support staff of AQRD, and Stewart Cober for
the management of the study. The project was supported by the Climate Change
and Air Quality Program (CCAP) and the Joint Oil Sands Monitoring program
(JOSM).
Edited by: Jennifer G. Murphy
Reviewed by: two anonymous referees